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Single-Molecule Fluorescence Spectroscopy Studies of the Microenvironment of Graded Amine-Functionalized Silica Surfaces
1 , * 1 , * 2
1  Kansas State University
2  Miami University

Published: 09 November 2020 by MDPI in The 1st International Electronic Conference on Catalysis Sciences session Posters

Single-molecule fluorescence spectroscopy was used to study the behavior of a 2-hydroxy substituted nile red (NR-OH) dye in silica films incorporating amine gradients. These amine gradients were made by the vapor-phase deposition of 3-aminopropyltrimethoxysilane (3-APTMOS) onto microporous silica base layers in a humidity-controlled chamber. Water contact angle and ellipsometry measurements were used to confirm the formation of amine gradients. X-ray photoelectron spectroscopy (XPS) measurement of the nitrogen content of the gradient films showed that there was a gradual decrease in the amine content of the gradient films away from the 3-APTMOS reservoir. Bulk fluorescence studies of the dye in basic media of varying strengths showed that there is a hypsochromic shift in the emission maxima of the spectra with increasing basicity. Two-color fluorescence videos acquired along the gradients showed that the fluorescence of the dye was more intense in the 580/40 nm channel (where the deprotonated form of the dye is more prevalent) than the 640/40 nm channel at the high amine end of the films. With decrease in the amine content of the films, the fluorescence of the 580/40 nm channel decreases relative to the 640/40 nm channel. This study can aid in understanding the nature of active sites of bifunctional catalysts for an aldol condensation reaction carried out at the single molecule level.

Keywords: fluorescence spectroscopy; single-molecule; catalysis; characterization
Comments on this paper
Violeta-Carolina Niculescu
Thank you for sharing this study
I also work with mesoporous silica functionalized or not with amino groups as adsorbents of dyes (such as Nile Blue, Rhodamine or Methyl Orange) from various wastes.
In this respect, in order to monitor the degree of adsorption, I used an usual UV-VIS spectrophotometer (please see: sciforum-033360), but also a dual fluorescence-Vis system, HORIBA DUETTA spectrophotometer.
So, I was thinking maybe we can collaborate for further research.
Thank you again for your presentation.
Best regards!
Dr. Violeta-Carolina NICULESCU, PhD in Chemistry
National Research and Development Institute for Cryogenic and Isotopic Technologies - ICSI Rm.Valcea, Romania

Christoffer Turner
interesting study
Great example of state-of-the-art characterization. The single-molecule spectroscopy can serve as an excellent tool for precisely probing reactivity.

Keith Hohn
Response to Violeta-Carolina

Thank you for your comment. It does appear that we have some similar research interests. One of the things we've noticed is that the spectra we get for our dyes is different in solution vs. on the catalyst. Do you ever notice anything like this?

Violeta-Carolina Niculescu
Hello again,
Thank you for your replay. Regarding the dye in the solution and the dye adsorbed on the catalyst, indeed, we noticed such modification. We supposed that these differences came from the fact that the chromophore groups of the dyes interact with the amino groups from the silica surface. As a consequence, the dyes on the catalyst present a more or less shift spectra. We put in evidence these interactions with FTIR, by diminution or disappearance of the typical vibrations of the
organic groups from the dye and of amino groups from silica.